Relief efforts for tsunami-hit countries are moving into a "second phase" of long-term projects to rebuild infrastructure and jobs instead of simply providing emergency aid, the UN's top tsunami coordinator says.

Margareta Wahlstrom says humanitarian groups now need fewer emergency airdrops because many roads are repaired, along with much other key infrastructure in hard-hit areas.

"We are now reaching very quickly the second phase of our relief operations, which will focus on how to support people to regain their livelihoods," Ms Wahlstrom said.

She says the new phase is focussing on "how to get (survivors) back to work, how to make sure the children get back to school, to prevent outbreaks of diseases."

Ms Wahlstrom says the international military aid effort, which has helped airlift urgently needed supplies, is winding down as civilian-led reconstruction efforts swung into full force.

"Three weeks down the line, I think now we are quite impressed with how rapidly the recovery efforts... are starting to take place," she said.

"In a number of weeks from now - it's not firmly set yet but possibly in another month - the military will be able to phase out."

Meanwhile, the World Health Organisation (WHO) says it is closely watching for any outbreak of diseases carried by mosquitos, such as malaria and dengue fever, after the tsunami.

The WHO's Thailand representative William Aldis says the risks have grown because there are "more mosquito bites because more people are in less secure areas, and mosquito breeding might be increased due to standing water in damaged areas."

He says the organisation is closely monitoring a slight rise in dengue infection rates in tsunami-hit parts of Thailand.

A massive undersea earthquake off the coast of Indonesia on December 26 triggered deadly tsunamis that smashed into Indian Ocean coastlines, killing more than 220,000 people.